The vibrational relaxation of a charged solute probes the vibrational density of states at oxide/water interfaces

Abstract

The vibrational lifetime of solute molecules is predicted to be slower at interfaces; however, ultrafast measurements show that this behavior can vary dramatically depending on interfacial structure and vibrational coupling pathways. Surprisingly, the nitrile stretch of SCN− exhibits unexpectedly rapid vibrational energy relaxation at aqueous (D2O) alumina interfaces, while there is no appreciable difference in relaxation in bulk and interfacial H2O. For interfacial D2O, the CN stretch lifetime is nearly three times shorter than in bulk D2O (T1 ∼ 22 ps). Ab initio simulations reveal an increased vibrational density of states (VDOS) at the low frequency OD stretch region compared to bulk D2O, enhancing overlap between SCN− and D2O vibrational modes. Additional factors—including stronger transition dipole–transition dipole coupling arising from reduced dielectric screening and increased orientational ordering of interfacial molecules—further accelerate vibrational relaxation at the interface. To directly probe how interfacial VDOS varies with surface structure, we employed the CN stretch lifetime of SCN− as a reporter of the O–D VDOS at two model alumina surfaces: Al2O3(0001)/D2O and Al2O3(112¯0)/D2O. IR pump–vibrational sum frequency generation (vSFG) probe measurements show a shorter vibrational lifetime at the Al2O3(0001)/D2O interface (7.1 ps) compared to the Al2O3(112¯0)/D2O interface (8.7 ps). Molecular dynamics simulations support these findings, showing that the low-frequency O–D stretch VDOS at the Al2O3(0001)/D2O interface is approximately ∼1.2 times higher than at the Al2O3(112¯0)/D2O surface. The higher VDOS provides more available accepting states for vibrational energy transfer, thus shortening the vibrational lifetime. Together, these results demonstrate that vibrational lifetimes of interfacial solutes provide a powerful experimental probe of interfacial VDOS and solute–solvent coupling. This approach offers new insight into vibrational relaxation pathways and the microscopic origins of energy dissipation in the bulk and at interfaces.